Question for an electronics buff

[Guest]

I'm almost ready to setup my refrigeration rig.. Someone asked me what I was going to do about "pin warming" on the CPU.. I decided to make a small heating coil to put on it similar to the one supplied with a Vapochill.. I got it to work.. Too good infact! If I hook it up to the 3.3v line of the PSU it gets hot enough to melt the plastic I have it enclosed in!

My question is this.. If I want to drop the heating coil temp, is this correct?

Watts = Amps x Volts, and Amps = Volts / Resistance
So if I give it 5V and I use a 2ohm resistor, I should get
12.5 Watts of heat??

I hope I'm doing this right & not just looking like an idiot.

 

[peteb]

you need to work out the resistance of the coil.

once you have that you can work out the required resistor needed.


Let's say the coil is 1 ohm. What power do you want? If you want 12.5W at the coil you need to generate a current of SQR 12.5 (I^2R=W) = 3.53A in the circuit. If you are using a 5V feed, then by V=IR you need to drop 3.53V over your 1 ohm resistor and the remaining potential over the second, unknown resistor.

Right, you have a 5V supply, and you want to supply 3.53V, which means you must drop 1.47V on the second resistor. Current in the circuit is 3.53A and so by V=IR again, we can deduce your resistor needs to be .42 ohms. Of course do not forget it needs to be fairly beefy since it is dropping about 1.66W itself.

At least I think that's right - it was a long time ago....

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[Guest]

Ok, I hooked up a .47 ohm, 5 watt resistor inline with the heating coil.. I don't have a good enough meter to check the resistance of the coil though.. It's just one strand of an 18 gauge wire about 10" long. It seemed to work great on a 300W PSU.. It was hot, but not hot enough to melt the plastic around it. The .47 ohm resistor was burning hot!
I tried it on the 400W PSU I'll be using in the PC and the coil was way too hot! I added another .47 ohm resistor just after the first and now it's at the right temp again.. Hot, but not too hot.. But the resistors are red hot.. Does this sound right? The PSU says it does 30A max at +5v.. I'm not really up on my electronics anymore (haven't used it since high school.. Don't use it, ya lose it!)
Is there a better way to get a hot heating coil, but no heat elsewhere?

 

[peteb]

hey - the assumption on the coil at 1 ohm was just that, an assumption. Like I said, if the coil needs that power drop, then the resistors will have to take the rest and need to be rated to handle the power. A 1W resistor will usually be encased in a heatsink....

You need to use resistor wire which has a know resistance per length. Using that to build your coil and heating element you can accurately work out your power drops and such.

If you get a length that would correspond to the correct resistance to generate the correct power outpun in a single wire, then you are fine. If you use a 5V line and want a 12.5W output then a coil setup will need 2.5A current (W=VA => 12.5=5A => A=2.5) so you have a 2.5A current and 5V supply therefore the coil needs a resistance of 2 Ohms to drop all the potential of a 5V line and generate 12.5W of heat. 12.5W is pretty hot though, you may find that you need to experiment a little.

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[Guest]

I guess I'll just stick to this setup then.. The resistors I'm using are encased in ceramic (they're rated at 5W). The coil seems to be holding up fine, so as long as the resistors can take it, it looks like it'll work.. Thanks for the help.

 

[Guest]

I'm giving myself a headache with this thing! I wish I had kept all my high school notebooks!

Ok, I bought a good multimeter & determined that the coil is only .5 ohms.
I have (2) .47 ohm, 5watt resistors inline with the coil
I checked the current & it reads 2.5A at 5VDC
so my total wattage is about 12.5W right?
Ok, now how to I determine how much wattage each resistor is getting? the 12.5W should be split equally into 3 (1 for each resistor since they're all about .5 ohms right)
so each point is getting about 4 watts?)
If this is right then all is well.. the heating coil seems hot enough & the resistors are rated at 5W so I should be ok. Thanx again.